Plasticity is the quality of being easy to shape: you push it, it takes a new shape, and it stays that way until someone changes it again. Warm wax has plasticity; a stone does not. Scientists also use this word a lot about the brain. They say the brain has plasticity when it can still change, make new paths, and keep learning — even after you grow up. That is the same idea, stretched: the brain is not fixed like stone. In talk about metals and materials it has a tighter meaning: the stuff bends and keeps the new shape, instead of springing back. The everyday word plastic sits under all of this — not plastic as in a shopping bag, though actually that name came from the same old idea of something you can mould.